However, performance evaluations based on the benchmark circuit have not been
done for these novel devices. Furthermore, the pass transistor is still used in the
novel FPGAs for routing signals.
This paper describes atomic-switch based FPGA. Atomic switch, which features
non-volatility, high on/off conductance ratio, and small foot print, replaces both
SRAM and pass transistor, resulting in reducing chip size and enhancing energy
efficiency. In the following Sect. 2, concept of “switch over the logic” is discussed.
And in Sect. 3, atomic switch technology is disclosed. Its operation principle and
features are described. And then, atomic-switch based FPGA will be described in
Sect. 4. Circuit architecture and performance evaluation using benchmark circuit
will be disclosed. Operation in harsh environments is discussed in Sect. 5. Finally, in
Sect. 6, an atomic-switch based FPGA as an accelerator of CPU is demonstrated,
where FPGA core is integrated with CPU core monolithically.
2 Concept of “Switch Over Logic”
Vertical structure of integrated circuit (IC) is divided into two parts: a transistor layer
and a Cu interconnect layer. In FPGA, both logic block and programming circuitry
of SRAM and pass transistors share the transistor layer. Stacking the programming
circuitry on top of the logic blocks closes the performance gap between FPGA and
ASIC (see Fig. 1). The concept of “switch over the logic” minimizes the area of
programmable logic [6]. Reduced length, parasitic capacitance, and resistance of the
interconnect improve a signal delay and dynamic power consumption. Implementing
configuration memory into the interconnect layer is achieved by using MRAM [3],
ReRAM [4] or PCM [5]. Further, stacking both the configuration memory and
switch over the logic block significantly improves logic density and performance.
Atomic switch is a promising candidate for this purpose. The non-volatility also
conserves stand-by power, while an SRAM-based FPGA is suffering from gate
tunneling current and source-drain leakage in SRAM.
3 Atomic Switch Technology
The atomic switch is one of ReRAMs, but it has a high on/off conductance ratio. The
metal filament formed between two electrodes results in a very low resistive state.
Atomic switch was originally developed by Terabe [7]. The developed atomic
switch has a nanometer-scale air-gap where the metal filament is formed electrically.
The silver electrode coated with silver sulfide is separated from Pt electrode by a
small air gap. The silver sulfide is an ionic conductor. At this small gap, a silver
filament forms via electrochemical reaction. By applying a voltage pulse, the
conductance increases step by step, of which conductance is quantized in units of
2e
2 /h. This indicates that the switching can be done by a few atoms.
Pathway to Atomic-Switch Based Programmable Logic
19
done for these novel devices. Furthermore, the pass transistor is still used in the
novel FPGAs for routing signals.
This paper describes atomic-switch based FPGA. Atomic switch, which features
non-volatility, high on/off conductance ratio, and small foot print, replaces both
SRAM and pass transistor, resulting in reducing chip size and enhancing energy
efficiency. In the following Sect. 2, concept of “switch over the logic” is discussed.
And in Sect. 3, atomic switch technology is disclosed. Its operation principle and
features are described. And then, atomic-switch based FPGA will be described in
Sect. 4. Circuit architecture and performance evaluation using benchmark circuit
will be disclosed. Operation in harsh environments is discussed in Sect. 5. Finally, in
Sect. 6, an atomic-switch based FPGA as an accelerator of CPU is demonstrated,
where FPGA core is integrated with CPU core monolithically.
2 Concept of “Switch Over Logic”
Vertical structure of integrated circuit (IC) is divided into two parts: a transistor layer
and a Cu interconnect layer. In FPGA, both logic block and programming circuitry
of SRAM and pass transistors share the transistor layer. Stacking the programming
circuitry on top of the logic blocks closes the performance gap between FPGA and
ASIC (see Fig. 1). The concept of “switch over the logic” minimizes the area of
programmable logic [6]. Reduced length, parasitic capacitance, and resistance of the
interconnect improve a signal delay and dynamic power consumption. Implementing
configuration memory into the interconnect layer is achieved by using MRAM [3],
ReRAM [4] or PCM [5]. Further, stacking both the configuration memory and
switch over the logic block significantly improves logic density and performance.
Atomic switch is a promising candidate for this purpose. The non-volatility also
conserves stand-by power, while an SRAM-based FPGA is suffering from gate
tunneling current and source-drain leakage in SRAM.
3 Atomic Switch Technology
The atomic switch is one of ReRAMs, but it has a high on/off conductance ratio. The
metal filament formed between two electrodes results in a very low resistive state.
Atomic switch was originally developed by Terabe [7]. The developed atomic
switch has a nanometer-scale air-gap where the metal filament is formed electrically.
The silver electrode coated with silver sulfide is separated from Pt electrode by a
small air gap. The silver sulfide is an ionic conductor. At this small gap, a silver
filament forms via electrochemical reaction. By applying a voltage pulse, the
conductance increases step by step, of which conductance is quantized in units of
2e
2 /h. This indicates that the switching can be done by a few atoms.
Pathway to Atomic-Switch Based Programmable Logic
19